Resistors play a large part in almost all electronic circuits. In many cases the performance of a circuit is limited by the accuracy of the resistors which are available to implement the circuit. Complementary metal oxide semiconductor (CMOS) chip manufacturing processes are not currently capable of realizing precise resistance values. For example, values of resistors implemented in CMOS chips can vary by as much as 20-30% of their designed values.
Digitally controllable resistors, when implemented in CMOS to counter this probabilistic spread in CMOS resistor yields, rely on transistor switches to change their value according to control signals. However, even in their “on” state, these switches introduce some “on-resistance” in the signal path which may change the behavior of the circuit. Traditional methods try to reduce the effect of this on-resistance by increasing the channel width of the transistors in the switch, hence reducing their on-resistance. However, this also increases the parasitic capacitance of the switch. Thus, CMOS switches either have high parasitic capacitance or significant on-resistance, both of which may affect the performance of the digitally controllable resistors and/or circuit in which they are used.
These issues pose a problem in manufacturing precise resistor values on-chip, whereas the current growth of the telecommunications industry requires the manufacturers to include as much functionality on-chip as possible and avoid using off-chip components. Hence a method to implement precise, linearly variable on-chip resistance values is needed. In addition, temperature changes in electronic circuits during use cause a drift in the values of on-chip resistors. In order to combat this tendency, on-chip variable resistors that can be tuned reliably and accurately within a specified range are also needed.
Several existing approaches attempt to address these problems, some examples of which will now be described. For example, trimming is a post-processing (i.e., post manufacturing) step used to correct the values of on-chip passive components. However this processing adds greatly to the cost of the finished chip. Another approach involves using MOS transistors as variable resistors by biasing and sizing them appropriately. However, this approach is not suitable for applications where, for example, a linear/constant resistance step is needed for every increment in the digital control word because the parallel connection of binary weighted transistors results in non-linear resistance steps in the active resistance range.
A third approach used to address these problems with on-chip resistors involves using pulse width modulation (PWM) on a field effect transistor (FET) in series with a primary resistor. However, this approach has a drawback for communication systems given the possibility of additional noise due to clock feed through. Yet another approach is to use MOS transistors as active fuses to short out tuning resistors placed in series or parallel. However, this approach is not suitable for CMOS applications since implementing low-resistance switches consumes a large area on the chip and introduces considerable parasitic capacitance in the resistor, which may induce non-linear behavior.
Still another approach involves using grounded switched resistor strings. However, this technique causes constant current consumption in the variable resistor due to the ground terminals. This makes this approach unattractive for use in single ended and/or low power circuits. In addition, the number of passive (resistors) and active (switches) components in the circuit increases in an exponential manner as the number of bits in the digital control word increases linearly. Yet another approach uses a CMOS switch or transmission gate arrays as variable resistors. However, this approach uses a binary weighted structure resulting in non-linear resistance steps. Additionally, the transmission gate has non-linear voltage over current characteristics near the extremes of supply voltage range which may lead to a decrease in usable voltage swing.
Accordingly, it would be desirable to provide digitally controllable resistor methods and devices which achieve arbitrarily small, yet substantially linear, incremental resistance steps irrespective of the on-resistance associated with the switches.